Microfluidic devices and methods for their preparation and use

ABSTRACT

A microfluidic device is provided. The microfluidic device includes a microtube having a hollow core. The microfluidic device further includes a plurality of nanopores extending radially outwards from an inner surface of the microtube.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 U.S.C. §119(a) of India Application No. 2232/DEL/2014 filed on Aug. 6, 2014. The India Application is hereby incorporated by reference in its entirety.

BACKGROUND

Heat exchangers are used in as variety of applications such as catalytic converters, chemical reactors, phase-change systems, electronics cooling and refrigeration systems. Heat exchangers can be single phase systems that utilize a single-phase working fluid to transfer heat from a heat source or to a heat sink. Alternatively, heat exchangers can be multiphase systems with boiling, evaporation and condensation of one or more of the working fluids.

Miniaturized heat exchangers are desirable for applications such as cooling of electronic appliances and for thermal management of high heat flux handling systems. Commercially available technologies for manufacturing micro and nano structured heat exchangers are expensive and are time consuming. Additionally, the existing techniques do not provide control on characteristics such as the surface morphology, geometry, roughness distribution and the porosity of the formed structures.

SUMMARY

Briefly, in accordance with one aspect, as microfluidic device is provided. The microfluidic device includes a microtube having a hollow core. The microfluidic device further includes a plurality of nanopores extending radially outwards from an inner surface of the microtube.

In accordance with another aspect, a heat exchanger is provided. The heat exchanger includes a reservoir. The heat exchanger further includes a plurality of microfluidic devices contained in the reservoir. Each of the plurality of microfluidic devices includes a microtube with a hollow core and a plurality of nanopores extending radially outwards from an inner surface of the microtube.

In accordance with another aspect, a method of forming a microfluidic device is provided. The method includes performing a first anodization of an aluminum wire to form nanoporous alumina on the aluminum wire and etching nanoporous alumina from the aluminum wire. The method also includes performing a second anodization of the aluminum wire to form a nanoporous alumina layer having a plurality of nanopores formed on the aluminum wire. The method further includes dissolving an aluminum core of the nanoporous alumina layer inform the microfluidic device having a microtube with hollow core and the plurality of nanopores extending radially from an outer surface towards an inner surface of the microtube.

In accordance with another aspect, a method of transferring thermal energy is provided. The method includes circulating a first fluid through as hollow core of a microtube and circulating a second fluid over a plurality of nanopores extending radially outwards from an inner surface of the microtube. The method also includes transferring the thermal energy between the first fluid and the second fluid that flows over a nanoporous surface formed by the plurality of nanopores of the microtube.

In accordance with another aspect, as microfluidic device is provided. The method includes a microtube having a hollow core. The method also includes a plurality of nanopores extending radially outwards from an inner surface of the microtube. The diameter of each of the plurality of nanopores tapers linearly from the outer surface towards the inner surface of the microtube.

The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates an example microfluidic device.

FIG. 2 is a cross-sectional view of the microfluidic device of FIG. 1.

FIG. 3 illustrates an example configuration of a heat exchanger.

FIG. 4 illustrates an example flow diagram of an embodiment of a method of forming a microfluidic device.

FIG. 5 illustrates an example flow diagram of an embodiment of a method of transferring thermal energy using the microfluidic device of FIG. 1.

FIG. 6 illustrates SEM micrographs of nanoporous alumina microtubes.

FIG. 7 illustrates an example SEM image of an alumina microtube having a separation layer.

DETAILED DESCRIPTION

In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be used, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the Figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein.

It will also be understood that any compound, material or substance which is expressly or implicitly disclosed in the specification and/or recited in a claim as belonging to a group or structurally, compositionally and/or functionally related compounds, materials or substances, includes individual representatives of the group and all combinations thereof. While various compositions, methods, and devices are described in terms of “comprising” various components or steps (interpreted as meaning “including, but not limited to”), the compositions, methods, and devices can also “consist essentially of” or “consist of” the various components and steps, and such terminology should be interpreted as defining essentially closed-member groups.

Some embodiments are generally directed to techniques of forming microfluidic devices such as used in heat exchangers. The present technique provides microfluidic devices formed of chemically inert and stable materials that facilitate heat transfer in chemically demanding environments. In some embodiments, the microfluidic device includes a nanoporous microtube of alumina. Such devices may be used for high temperature applications. The present technique also provides an efficient process of forming the microfluidic devices using anodization and chemical etching. In some embodiments, an array of the microfluidic devices may be arranged and operated in parallel for heat exchange applications. The microfluidic devices described below may be used for a variety of applications such as electronic thermal management, chemical sensing and as a fluid transport manipulator.

Referring now to FIG. 1, an example microfluidic device 100 is illustrated. The microfluidic device 100 includes a microtube 102 having as hollow core 104. The microfluidic device 100 also includes a plurality of nanopores (generally represented by reference numeral 106) extending radially outwards from an inner surface 108 of the microtube 102. The microtube 102 is formed of a chemically inert material such as alumina (Al₂O₃), titania (TiO₂), silicon (Si), silica (SiO₂), or combinations thereof.

In some embodiments, a first fluid flows within the hollow core 104 of the microtube 102 and a second fluid flows over a nanoporous surface formed by the plurality of nanopores 106. In one example, the microfluidic device 100 is configured to have the first fluid at a first temperature and the second fluid at a second temperature different than the first temperature. The plurality of nanopores 106 are configured to facilitate heat transfer between the first fluid and the second fluid. In this embodiment, the flow of the second fluid within the nanopores 106 enhances the contact area and substantially reduces the heat transfer distance.

In some examples, the plurality of nanopores 106 have tapered diameter that is larger at an outer surface 110 towards the inner surface 108 of the microtube 102. In one example, the plurality of nanopores 106 have diameter that tapers linearly from the outer surface 110 towards the inner surface 108 of the microtube 102. The diameter of the plurality of nanopores 106 may be selected to control the physical morphology of the microfluidic device 100.

In some examples, the surface of the nanopores 106 is modified to enhance wettability of the nanopores 106. For example, the surface of the nanopores 106 is coated with a surface energy modifier material to enhance the wettability of the nanopores 106. Examples of the surface energy modifier material include amines, fluorinated polyurethanes, polyethers, sodium polyacrylates ((C₃H₃NaO₂)n), ammonium polyacrylate, fatty ethoxalates, hydropalat, poly alkyl ammonium, benzyl phosphoric acids, pyridine based modifiers, or combinations thereof.

FIG. 2 is a cross-sectional view of the microfluidic device 100 of FIG. 1. As illustrated, the microfluidic device 100 includes the microtube 102 having the hollow core 104. The microtube 102 is formed of a chemically inert material such as alumina (Al₂O₃), titania (TiO₂), silicon (Si), silica (SiO₂), or combinations thereof. The microtube 102 may generally be of any size. In some examples, the microtube 102 has an outer diameter of about 30 microns (μm) to about 300 μm. Specific examples of the outer diameter of the microtube 102 include about 30 μm, about 50 μm, about 100 μm, about 150 μm, about 200 μm, about 250 μm, about 300 μm and ranges between any two of these values (including endpoints).

In some examples, the microtube 102 has a wall thickness of about 10 μm to about 50 μm. Specific examples of the wall thickness of the microtube 102 include about 10 μm, about 15 μm, about 20 μm, about 25 μm, about 30 μm, about 35 μm, about 40 μm, about 45 μm, about 50 μm, and ranges between any two of these values (including endpoints).

In some examples, the hollow core 104 of the microtube 102 has a diameter of about 1 μm to about 100 μm. Specific examples of diameter include about 1 μm, about 10 μm, about 20 μm, about 40 μm, about 60 μm, about 80 μm, about 100 μm, and ranges between any two of these values (including endpoints).

In this embodiment, the microfluidic device 100 includes the plurality of nanopores 106 extending radially outwards from an inner surface 108 of the microtube 102. The plurality of nanopores 106 may generally be of any size. In some examples, the plurality of nanopores 106 have an average diameter of about 5 nanometers nm to about 150 nm. Specific examples of diameter of the nanopores 106 include about 5 nm, about 10 nm, about 20 nm, about 40 nm, about 50 nm, about 60 nm, about 80 nm, about 100 nm, about 130 nm, about 150 nm and ranges between any two of these values (including endpoints).

In some examples, the plurality of nanopores 106 have a wall thickness of about 5 nm to about 70 nm. Specific examples of the wall thickness of the nanopores 106 include about 5 nm, about 15 nm, about 30 nm, about 45 nm, about 60 nm, about 70 nm and ranges between any two of these values (including endpoints).

In this example, the microtube 102 includes a separation layer 202 disposed proximate the inner surface 108 of the microtube 102. The plurality of nanopores 106 terminate at the separation layer 202 to prevent mixing of the first fluid and the second fluid. The separation layer 202 facilitates physical segregation between the first fluid and the second fluid and also minimizes the distance between the first fluid and the second fluid. In some examples, the separation layer 202 is formed of a fluid-impermeable material. Examples of the fluid-impermeable material include, but are not limited to, alumina (Al₂O₃), titania (TiO₂), silicon (Si), silica (SiO₂), or combinations thereof.

The separation layer 202 can generally be of any thickness. In sonic examples, the separation layer 202 has a thickness of about 10 nm to about 80 nm. Specific examples of the thickness of the separation layer 202 include about 10 nm, about 20 nm, about 30 nm, about 40 nm, about 50 nm, about 60 nm, about 70 nm, about 80 nm and ranges between any two of these values (including endpoints).

The microfluidic device 100 facilitates a re-entrant behavior allowing the entrapment of a small amount of vapor that facilitates nucleation. The microfluidic device 100 described above may be used in a variety of applications. For example, the microfluidic device 100 may be configured for use as a heat exchanger, a microreactor, a chemical sensor, a fluid transport manipulator, or combinations thereof.

In one example, the microfluidic device 100 may be configured for use in a microreactor. In this example, the plurality of nanopores 106 are configured to introduce a reactant inside the hollow core 104 of the microtube 102 to facilitate mixing of the reactant with another reactant. In some examples, where mixing of two fluids is desired, the separation layer 202 may be removed entirely by a reverse anodization (or cathodization) process using phosphoric acid.

FIG. 3 illustrates an example configuration of a heat exchanger 300. The heat exchanger 300 includes a reservoir 302 configured to contain a first fluid 304. The heat exchanger 300 also includes a plurality of microfluidic devices 306 contained in the reservoir 302. Each of the plurality of microfluidic devices 306 has a configuration such as described with reference to FIGS. 1 and 2. In particular, each of the plurality of microfluidic devices 306 includes a microtube 102 with a hollow core 104 and a plurality of nanopores 106 extending radially outwards from an inner surface of the microtube 102. In this example, the plurality of microfluidic devices 306 are formed of alumina (Al₂O₃), titania (TiO₂) silicon (Si), silica (SiO₂), or combinations thereof.

In this example, the first fluid 304 flows within the hollow core 104 of the microfluidic devices 306. Moreover, a second fluid 308 flows over a nanoporous surface formed by the plurality of nanopores 106. In this example, the heat exchanger 300 includes an inlet 310 configured to receive the second fluid 308 and an outlet 312 configured to discharge the second fluid 308.

In this example, the plurality of microfluidic devices 306 are fluidically coupled to the inlet 310 and the outlet 312 using an elastomer 314. Examples of the elastomer include thermoset ceramic, resins, plastic, or combinations thereof. In some examples, the plurality of microfluidic devices 306 may be embedded in the elastomer 314 such that the elastomer 314 sets around the plurality of microfluidic devices 306.

In some examples, the first fluid 304 has a first temperature and the second fluid 308 has a second temperature different than the first temperature. The plurality of microfluidic devices 306 facilitate heat transfer between the first fluid 304 and the second fluid 308. For example, the microfluidic devices 306 may be immersed in a cold fluid and the fluid flowing over a nanoporous surface formed by the nanopores of the microfluidic devices 306 may be a hot fluid. The two fluids exchange heat across the nanoporous surface of the microtube 102.

The nanoporous microfluidic devices 306 facilitate heat transfer in the heat exchanger 300 where the evaporative cooling occurs at the nanoscale lengths. In some examples, the individual microfluidic devices 306 may be used as a heat exchanger in micro-fluidic applications and macroscopic large scale applications. In some examples, a large number of such parallel microfluidic devices 306 carrying a second fluid (e.g., a hot fluid) and immersed in the first fluid (e.g., a cold fluid) may form a macroscopic heat exchanger 300 that may facilitate exchange of substantial amounts of energy.

FIG. 4 illustrates an example flow diagram 400 of an embodiment of a method of forming a microfluidic device. At block 402, an aluminum wire is cleaned and was vacuum annealed at a temperature of about 500° C. and pressure of about 10⁻⁵ mbar. In this embodiment, the aluminum wire is cleaned with a solution of acetone. In some examples, the purity of the cleaned aluminum wire may be about 99.99 to 99.9995 percent (%).

At block 404, the cleaned aluminum wire is electropolished in presence of an electropolishing solution. Examples of the electropolishing solution include, but are not limited to, perchloric acid (HClO₄), ethanol (CH₃CH₂OH), sulfuric acid (H₂SO₄), phosphoric acid (H₃PO₄), or combinations thereof. In this embodiment, the cleaned aluminum wire is electropolished at a current density of about 0.076 Ampere per square centimeter (A/cm²) to about 0.17 A/cm². Specific examples of the current density include about 0.076 A/cm², about 0.086 A/cm², about 0.096 A/cm², about 0.10 about 0.17 A/cm², and ranges between any two of these values (including endpoints).

In this embodiment, the cleaned aluminum wire is electropolished for a time period of about 30 seconds to about 5 minutes. Specific examples of the time period include about 30 seconds, about 1 minute, about 2 minutes, about 3 minutes, about 4 minutes, about 5 minutes, and ranges between any two of these values (including endpoints).

At block 406, at last anodization of the aluminum wire is performed to form nanoporous alumina on the aluminum wire. In this embodiment, the first anodization of the aluminum wire is performed in a solution of oxalic acid (H₂C₂O₄), sulfuric acid (H₂SO₄), phosphoric acid (H₃PO₄), chromic acid (H₂CrO₄), or combinations thereof. Further, the first anodization of the aluminum wire is performed at an anodizing voltage of about 10 Volts (V) to about 1.20V. Specific examples of the anodizing voltage include about 10V, about 30V, about 50V, about 70V, about 90V, about 100V, about 120V, and ranges between any two of these values (including endpoints).

Here, the first anodization of the aluminum wire is performed for a time period of about 1 hour to about 4 hours. Specific examples of the time period include about 1 hour, about 2 hours, about 3 hours, about 4 hours and ranges between any two of these values (including endpoints).

At block 408, the nanoporous alumina is etched from the aluminum wire. In one example embodiment, the etching of the nanoporous alumina is carried out using a mixture of chromic acid (H₂CrO₄) and phosphoric acid (H₃PO₄). Here, the etching of the nanoporous alumina is performed at a temperature of about 90 degree centigrade (° C.) to 100° C. Specific examples of the temperature include about 90° C., about 92° C., about 94° C., about 96° C., about 98° C., about 100° C., and ranges between any two of these values (including endpoints). In one example, the etching of the nanoporous alumina is performed at a temperature of about 90° C.

At block 410, a second anodization of the aluminum wire is performed to form a nanoporous alumina layer having a plurality of nanopores formed on the aluminum wire. In this embodiment, the second anodization of the aluminum wire is performed in a solution of oxalic acid (H₂C₂O₄), sulfuric acid (H₂SO₄), phosphoric acid (H₃PO₄), chromic acid (H₂CrO₄), or combinations thereof. Further, the second anodization of the aluminum wire is performed at an anodizing voltage of about 10 Volts (V) to about 120V. Specific examples of the anodizing voltage include about 10V, about 30V, about 50V, about 70V, about 90V, about 100V, about 120V, and ranges between any two of these values (including endpoints). Here, the second anodization of the aluminum wire is performed for a time period of about 2 hours to about 5 hours. Specific examples of the time period include about 2 hours, about 3 hours, about 4 hours, about 5 hours, and ranges between any two of these values (including endpoints).

In some examples, the anodization time may be selected to control a thickness of the nanoporous alumina layer. In some examples, the thickness of the nanoporous alumina layer may be reduced by decreasing the anodizing voltage. For example, the anodizing voltage may be decreased gradually for a selected anodization time (e.g., about 5 minutes) to reduce the thickness of the nanoporous alumina layer.

At block 412, an aluminum core of the nanoporous alumina layer is dissolved to form the microfluidic device. The formed microfluidic device includes a microtube with hollow core and a plurality of nanopores extending radially from an outer surface towards an inner surface of the microtube. In this embodiment, the aluminum core is dissolved using cupric chloride (CuCl₂) solution, mercury chloride (HgCl₂) solution, ferric chloride (FeCl₃) solution, or combinations thereof.

In some examples, the anodizing voltage is reduced to control the thickness of the separation layer proximate the inner surface of the microtube. In this example, the separation layer is formed of alumina. The separation layer substantially prevents mixing of a fluid inside the microtube with a fluid outside through the nanopores.

The separation layer may be etched in as solution of phosphoric acid (H₃PO₄). The etching time may be adjusted to completely etch away the bottom layer of the nanoporous alumina. In some examples, the etching time is about 60 minutes to about 100 minutes. Specific examples of the etching time include about 60 min, about 70 min, about 80 min, about 90 min, about 100 min, and ranges between any two of these values (including endpoints). The etching time varies as the barrier layer is partially or completely removed.

In some examples, metal wires are deposited within the plurality of nanopores of the microtube after performing the second anodization of the aluminum wire. Examples of the metal wires include, but are not limited to copper (Cu), silver (Ag), gold (Au), nickel (Ni), or combinations thereof.

In one example, the metal wires are electro-deposited within the plurality of nanopores of the microtube using about 0.5 M water solution of copper sulphate (CuSO₄.5H₂O). Here, the metal wires are deposited at about 0.06 A current for about 5 minutes. Further, the nanoporous alumina layer may be exposed to a solution of phosphoric acid (H₃PO₄) prior to the electro-deposition of the metal wires to remove an insulating impermeable aluminium oxide layer (Al₂O₃) within the nanopores. The etching results in formation of a metallic nano-brush structure with copper (Cu) nano-bristles.

The process of forming the microfluidic devices described above is scalable and may be used to form a plurality of devices that may be used in as variety of applications such as a heat exchanger, a microreactor, a chemical sensor, a fluid transport manipulator, or combinations thereof

FIG. 5 illustrates an example flow diagram 500 of an embodiment of a method of transferring thermal energy using the microfluidic device such as the device of FIG. 1. At block 502, a first fluid is circulated through a hollow core of a microtube. In this example, the microtube is formed of alumina. At block 504, a second fluid is circulated through a plurality of nanopores extending radially outwards from an inner surface of the microtube. In this embodiment, the first fluid is at as first temperature and the second fluid is at a second temperature different than the first temperature.

At block 505, the thermal energy is transferred between the first fluid and the second fluid through the plurality of nanopores of the microtube.

EXAMPLES

The present invention will be described below in further detail with examples and comparative examples thereof, but it is noted that the present invention is by no means intended to be limited to these examples.

Example 1 Formation of the Microfluidic Device

A microfluidic device was formed using the process of FIG. 4. Here, an aluminum wire was cleaned with a solution of acetone. The cleaned aluminum wire was electropolished in presence of perchloric acid (HClO₄) and ethanol (CH₃CH₂OH) at a current density of about 0.076 Ampere per square centimeter (A/cm²). The concentrations of the perchloric acid (HClO₄) and ethanol (CH₃CH₂OH) were about 1:5 fraction. Moreover, the cleaned aluminum wire was electropolished for about 1 minute.

Next, a first anodization of the aluminum wire was performed to form nanoporous alumina on the aluminum wire. Here, the first anodization of the aluminum wire was performed in about 0.3 molar (M) solution of oxalic acid (H₂C₂O₄), in de-ionized water. The first anodization of the aluminum wire was performed at an anodizing voltage of about 40 Volts (V) for a time period of about 3 hours.

The nanoporous alumina was then etched from the aluminum wire. The etching of the nanoporous alumina was carried out using as mixture of chromic acid (H₂CrO₄) and phosphoric acid (H₃PO₄). Here, the etching of the nanoporous alumina was performed at a temperature of about 90° C.

Subsequently, a second anodization of the aluminum wire was performed to form a nanoporous alumina layer having a plurality of nanopores formed on the aluminum wire. Here, the second anodization of the aluminum wire was performed in a solution of oxalic acid (H₂C₂O₄) at an anodizing voltage of about 40 Volts (V). Here, the second anodization of the aluminum wire was performed for a time period of about 3 hours. An aluminum core of the nanoporous alumina layer was dissolved using cupric chloride (CuCl₂) solution to form the microfluidic device having a microtube with hollow core and a plurality of nanopores extending radially from an outer surface towards an inner surface of the microtube.

The anodizing voltage was reduced to form a separation layer proximate the inner surface of the microtube and to control the thickness of the separation layer. The separation layer was formed of alumina.

Example 2 Characterization of the Nanoporous Alumina Microtubes

The nanoporous anodized alumina microtube formed using the process described in FIG. 4. was characterized by scanning electron microscopy (SEM). FIG. 6 illustrates SEM micrographs 600 of the nanoporous alumina microtubes.

The image 602 illustrates the microtube having a diameter of about 10 mm that was mounted on a SEM stage. An outer surface of the microtube is illustrated in image 604. Here, the diameter of the microtube was measured to be about 70 micrometers (μm). The image 606 illustrates a nanoporous surface of the microtube with the nanopores roughly arranged on a hexagonal lattice. The image 608 illustrates a cross-sectional view of the microtube. It was observed that the microtube expanded on breaking that was indicative of the microtube being under static stress. A cross-sectional view of the microtube is shown in image 610. The wall thickness of the microtube measured from the image 610 was about 12 microns.

The image 612 is a magnified view of the microtube. As can be seen from the image 612, the nanopores were observed to be extending from the outer surface of the microtube towards the inner surface of the microtube. Further, a magnified image of the nanopores is represented by reference numeral 614. As can be seen from the image 614, an increase in the diameter of the nanopores was observed with an increase in the radius of the microtube.

Image 616 illustrates another magnified view of the microtube with the nanopores. Here, the diameter of the microtube was measured to be in a range of about 35 nm to about 40 nm for the inner surface of the microtube. Image 618 illustrates the outer surface of the microtube. The diameter of the nanopores was measured to be about 90 nm at the outer surface of the microtube for the microtube having a thickness of about 10 mm.

Example 3 Scanning Electron Micrograph (SEM) of an Alumina Microtube Having a Separation Layer

The SEM image 700 illustrates the alumina microtube having a separation layer formed of impermeable alumina. As can be seen, the impermeable alumina layer was observed to be capping the nanopores at the inner surface of the microtube. Here, the thickness of the alumina layer was about 50 nm. The alumina layer was observed to substantially prevent the physical mixing of the inner and outer fluids.

Example 4 Wetting and Evaporation Characteristics of Water on Flat Nanoporous Alumina Surfaces

Wetting and evaporation characteristics of water on flat nanoporous alumina surface of the microfluidic devices were observed. It was observed that wetting occurred at a rapid rate on the flat surface and changed with morphology of the nanopores. Further, the evaporation of water from the nanoporous alumina surface was observed to be higher compared to evaporation from a non-porous surface having a similar chemical nature. Moreover, a wetting-dewetting transition was observed with the change in the diameter of the nanopores. Here, non-wetting behavior was observed for devices with nanopores having a diameter of less than about 50 nm. Further, wetting behavior was observed for devices with nanopores having a diameter of greater than about 50 nm.

It was also observed that the fluid inside the microtube of the microfluidic devices described above had enhanced thermal exchange rates with another fluid outside as compared to a commercially available heat pipe. Here, the heat exchange between two fluids occurs over a substantially thin layer (e.g., about 50 nm) as compared to heat exchange over about 100 micrometers in conventional heat pipes. Such efficient heat exchange was facilitated by the nanopores in the device.

The present disclosure is not to be limited in terms of the particular embodiments described in this application, which are intended as illustrations of various aspects. Many modifications and variations can be made without departing from its spirit and scope, as will be apparent to those skilled in the art. Functionally equivalent methods and apparatuses within the scope of the disclosure, in addition to those enumerated herein, will be apparent to those skilled in the art from the foregoing descriptions. Such modifications and variations are intended to fall within the scope of the appended claims.

The present disclosure is to be limited only by the terms of the appended claims, along with the full scope of equivalents to which such claims are entitled. It is to be understood that this disclosure is not limited to particular methods, reagents, compounds compositions or biological systems, which can, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting.

With respect to the use of substantially any plural and/or singular terms herein, those having skill in the art can translate from the plural to the singular and/or from the singular to the plural as is appropriate to the context and/or application. The various singular/plural permutations may be expressly set forth herein for sake of clarity.

It will be understood by those within the art that, in general, terms used herein, and especially in the appended claims (e.g., bodies of the appended claims) are generally intended as “open” terms (e.g., the term “including” should be interpreted as “including but not limited to,” the term “having” should be interpreted as “having at least,” the term “includes” should be interpreted as “includes but is not limited to,” etc.). It will be further understood by those within the art that if a specific number of an introduced claim recitation is intended, such an intent will be explicitly recited in the claim, and in the absence of such recitation no such intent is present.

For example, as an aid to understanding, the following appended claims may contain usage of the introductory phrases “at least one” and “one or more” to introduce claim recitations. However, the use of such phrases should not be construed to imply that the introduction of a claim recitation by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim recitation to embodiments containing only one such recitation, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an” (e.g., “a” and/or “an” should be interpreted to mean “at least one” or “one or more”); the same holds true for the use of definite articles used to introduce claim recitations.

In addition, even if a specific number of an introduced claim recitation is explicitly recited, those skilled in the art will recognize that such recitation should be interpreted to mean at least the recited number (e.g., the bare recitation of “two recitations,” without other modifiers, means at least two recitations, or two or more recitations). Furthermore, in those instances where a convention analogous to “at least one of A, B, and C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, and C” would include but not be limited to systems that have A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.). In those instances where a convention analogous to “at least one of A, B, or C, etc.” is used, in general such a construction is intended in the sense one having skill in the art would understand the convention (e.g., “a system having at least one of A, B, or C” would include but not be limited to systems that have. A alone, B alone, C alone, A and B together, A and C together, B and C together, and/or A, B, and C together, etc.).

It will be further understood by those within the art that virtually any disjunctive word and/or phrase presenting two or more alternative terms, whether in the description, claims, or drawings, should be understood to contemplate the possibilities of including one of the terms, either of the terms, or both terms. For example, the phrase “A B” will be understood to include the possibilities of “A” or “B” or “A and B.”

As will be understood by one skilled in the art, for any and all purposes, such as in terms of providing a written description, all ranges disclosed herein also encompass any and all possible sub ranges and combinations of sub ranges thereof. Any listed range can be easily recognized as sufficiently describing and enabling the same range being broken down into at least equal halves, thirds, quarters, fifths, tenths, etc. As a non-limiting example, each range discussed herein can be readily broken down into a lower third, middle third and upper third, etc.

As will also be understood by one skilled in the art all language such as “up to,” “at least” “greater than,” “less than,” and the like include the number recited and refer to ranges which can be subsequently broken down into sub ranges as discussed above. Finally, as will be understood by one skilled in the art, a range includes each individual member. Thus, for example, a group having 1-3 cells refers to groups having 1, 2, or 3 cells. Similarly, a group having 1-5 cells refers to groups having 1, 2, 3, 4, or 5 cells, and so forth.

While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims. 

1. A microfluidic device comprising: a microtube having a hollow core; and a plurality of nanopores extending radially outwards from an inner surface of the microtube.
 2. The microfluidic device of claim 1, further comprising: first fluid that flows within the hollow core; and a second fluid that flows over a nanoporous surface formed by the plurality of nanopores.
 3. The microfluidic device of claim 2, wherein the microfluidic device is configured to have the first fluid at a first temperature and the second fluid at a second temperature different than the first temperature and wherein the plurality of nanopores facilitate heat transfer between the first fluid and the second fluid.
 4. The microfluidic device of claim 1, wherein the microfluidic is formed of alumina (Al₂O₃), titania (TiO₂), silicon (Si), silica (SiO₂), or combinations thereof.
 5. The microfluidic device of claim 1, wherein the microtube has an outer diameter of about 30 microns (μm) to about 300 μm; the microtube has a wall thickness of about 10 μm to about 50 μm; the hollow core has a diameter of about 1 μm to about 100 μm; the plurality of nanopores have an average diameter of about 5 nanometers (nm) to about 150 nm; and the plurality of nanopores have a wall thickness of about 5 nm to about 70 nm.
 6. The microfluidic device of claim 1, wherein the microtube further comprises a separation layer disposed proximate the inner surface of the microtube, wherein the plurality of nanopores terminate at the separation layer to prevent mixing of the first fluid and the second fluid.
 7. The microfluidic device of claim 6, wherein the separation layer is formed of a fluid-impermeable material.
 8. The microfluidic device of claim 7, wherein the separation layer is formed of alumina (Al₂O₃), titania (TiO²), silicon (Si) and silica (SiO₂), or combinations thereof.
 9. The microfluidic device of claim 1, wherein the plurality of nanopores have tapered diameter larger at the outer surface than towards the inner surface of the microtube.
 10. A heat exchanger comprising: a reservoir; a plurality of microfluidic devices contained in the reservoir, wherein each of the plurality of microfluidic devices comprises: a microtube with a hollow core; and a plurality of nanopores extending radially outwards from an inner surface of the microtube.
 11. The heat exchanger of claim 10, further comprising: a first fluid contained in the reservoir, wherein the first fluid flows within the hollow core; and a second fluid that flows over a nanoporous surface formed by the plurality of nanopores.
 12. The heat exchanger of claim 11, further comprising: an inlet configured to receive the second fluid; and an outlet configured to discharge the second fluid.
 13. The heat exchanger of claim 11, wherein the microfluidic device is configured to have the first fluid at a first temperature and the second fluid at a second temperature different than the first temperature and wherein the plurality of nanopores of the microfluidic devices facilitate heat transfer between the first fluid and the second fluid.
 14. A method of transferring thermal energy, the method comprising: circulating a first fluid through a hollow core of a microtube; circulating a second fluid over a plurality of nanopores extending radially outwards from an inner surface of the microtube; and transferring the thermal energy between the first fluid and the second fluid that flows over a nanoporous surface formed by the plurality of nanopores of the microtube.
 15. The method of claim 14, wherein transferring the thermal energy comprises transferring the thermal energy between the first fluid at a first temperature and the second fluid at a second temperature different than the first temperature.
 16. The method of claim 14, wherein circulating the first fluid comprises circulating the first fluid through the microtube formed of alumina.
 17. A microfluidic device comprising: a microtube having a hollow core; and a plurality of nanopores extending radially outwards from an inner surface of the microtube, wherein the diameter of each of the plurality of nanopores tapers linearly from the outer surface towards the inner surface of the microtube.
 18. The microfluidic device of claim 17, further comprising a first fluid that flows within the hollow core of the microtube.
 19. The microfluidic device of claim 18, wherein the plurality of nanopores are configured to introduce a reactant inside the hollow core of the microtube to facilitate mixing of the reactant and the first fluid.
 20. The microfluidic device of claim 17, wherein the microtube is formed of alumina. (Al₂O₃), titania (TiO₂), silicon (Si), silica (SiO₂), or combinations thereof.
 21. The microfluidic device of claim 17, wherein the microtube has an outer diameter of about 30 microns (μm) to about 300 μm; the microtube has a wall thickness about 10 μm to about 50 μm; the hollow core has a diameter of about 1 μm to about 100 μm; each of the plurality of nanopores has a diameter of about 5 nanometers (nm) to about 150 nm; and each of the plurality of nanopores has a wall thickness of about 5 nm to about 70 nm. 